Radio receiving system



Sept. 17, 1940. K. KOSCHMIEDEfi 2,214,929

RADIO RECEIVING SYSTEM Filed Aug. 25,. 1938 HIGH FREQUENCY AMPLIFIER MODULATING STAGE INTERMEDIATE FREQUENCY AMPLIFIER LOW FREQUENCY AMPLIFIER MODULATING STAGE OSCILLATOR OSCILLATOR DELAY DEVICE hie/Mar: Karl K0 ac/Im (eder y filforney Lil Patented Sept. 17, 1940 RADIO RECEIVING SYSTEM- Kurt Koschmieder, Falkensee/Ost-Havelland,

Germany, assignor to C. Lorenz Aktiengesellschaft, many, a company BerlimTempelhof, Lorenzweg,

Ger-

, Application August 25, 1938, Serial No. 226,613

In Germany July 26, 1937 4 Claims.

The present invention relates to radio receiving systems, and more specifically, to a method of reducing noise voltages in such systems.

Radio receiving devices, and particularly direction finding receivers, frequently employ a local oscillator, the oscillations of which together with the received carrier frequency oscillations produce a low-frequency beat note which enables a reception from undamped transmitters and, moreover, involves a higher degree of amplification. In direction finding reception of modulated waves, e. g. as emitted from broadcasting transmitters, the local oscillator serves for eliminating the side bands, in order to more sharply define the minimum position, or, in cases that pointer indicating instruments are employed to suppress pointer oscillations introduced by modulation. It is of no importance when such local oscillator is used, whether the high frequency amplifier of the receiving device is an amplifier of the straight line type, or if an intenae-diate heterodyning frequency has been introduced, in which latter case the above mentioned local oscillator acts as second heterodyning oscillator in the output circuit of the intermediate frequency amplifier.

The invention has for its object to reduce noise voltages in the above mentioned type of receiving devices.

It is known from practical experience in connection with receiving devices operating with a local oscillator that the'noise voltage increases with increased amplitude of the local oscillator when the antenna is disconnected. In fact, the

sensitiveness of the receiver is simultaneously en-:

hanced, but not with the same rate as the noise voltage. Therefore, in the presence of faint input voltages in the receiver and a high voltage of the heterodyning oscillator, a high sensitiveness of the receiving device is secured, but this high sensitiveness is followed by a high noise voltage which overlaps the faint signals received, so that the high sensitiveness of the receiver becomes useless. On the other hand, if the voltage of the heterodyning oscillator is reduced, a noise voltage decrease is obtained, but the rate of this decrease differs from the rate with which the receiver sensitiveness diminishes. Consequently, an optimum ratio of the receiver sensitiveness to the noise voltage must exist.

According to the main feature of the invention, the voltage of the heterodyning oscillator is so regulated in dependency upon the voltage picked up across the input circuit of the receiver that the ratio of the receiver sensitiveness to the noise voltage continually assumes an optimum value.

This regulation may be accomplished manually, or, according to a further feature of the invention, automatically in a manner similar to the fading regulation of the heterodyning oscillator in response to incoming signals.

The invention will be more fully understood from the following description taken in con-- junction with the accompanying drawing, the single figure of which schematically illustrates one way of carrying out the invention.

The schematic illustration represents an intermediate frequency amplifier comprising a local oscillator of the heretofore mentioned type which serves for producing a low-frequency beat note. The antenna potential is supplied to a mixing or modulating stage. Mi through a first high frequency amplifier, VI, and the local oscillator OI delivers its intermediate frequency likewise to the mixing or moduating stage MI. The intermediate frequency is amplified in an intermediate frequency amplifier ZF and then impressed upon a second mixing or modulating stage M2, in which device a low-frequency beat note is produced in cooperation with the local oscillator 02. After rectification and low frequency amplification at V3, the beat note is impressed upon a telephone T. The voltage of the local oscillator 02 is in accordance with a further feature of the invention so regulated in dependency upon the input voltage of the receiver that continually an optimum ratio of the operating voltage to the noise voltage is maintained. This may be manually accomplished, or even automatically by branching off a portion of the received carrier frequency at the output side of the intermediate frequency amplifying stage ZF', for example. This portion of the intermediate frequency is then rectified in the rectifying device. G, whereupon the resulting direct current is utilized for controlling the grid bias of the valve forming part of the local oscillator 02. This control is effected in such manner that in the presence of a low input voltage the local oscillator produces an accordingly low voltage, While this local oscillator voltage assumes a higher value with an increased reception voltage. In cases that telegraph signals are to be received, a retardation or delay member D is interposed in the control circuit, and this member has for its object to eliminate the possibility that the automatic control means respond to the keying frequency of the telegraph signals.

The application of the present invention is of specific importance in connection with radio direction finding receivers which operate in accordance with the method of minimum position,

since this type of receivers must involve their utmost sensitiveness in the minimum position, and because of the fact that the noise voltage covers the distinctly defined minimum. The arrangement according to the invention heretofore described provides an optimum ratio of the receiver sensitiveness to the noise voltage, from which follows that the accuracy in direction finding is considerably enhanced.

What is claimed is:

1. In a radio receiver comprising an antenna, amplifier stages, and local oscillators, the method of reducing the effects of internal noise voltages in said receiver which comprises varying the heterodyning voltage of one of said oscillators in the. same sense as the voltage picked up by said antenna varies and at a rate such that the ratio of the receiver sensitiveness to said noise voltages is continually maintained at its optimum value.

2. In a radio direction finder receiver comprising an antenna, amplifier stages, and local oscillators, the method of reducing the effect of internal noise voltages in said receiver which comprises controlling the heterodyning voltage of one of said local oscillators by the voltage picked up by said antenna such that the ratio of the receiver sensitiveness to said noise voltages is continually maintained at its optimum value.

3. In a radio receiving system comprising an antenna, amplifier stages, and local oscillators comprising vacuum tubes each having a grid, the method of reducing the effect of internal noise voltages in said receiver which method consists in branching 01f one portion of the received carrier wave, rectifying said portion and utilizing said rectified portion for controlling the grid bias of one of said local oscillator tubes in such manner that the ratio of the receiver sensitiveness to the noise voltages continually assumes optimum value.

4. In a radio receiving system comprising an antenna, amplifier stages, and local oscillators comprising vacuum tubes each having a grid, the method of reducing noise voltages which method consists in branching off one portion of the received carrier wave, rectifying said portion, utilizing said rectifier portion for controlling the grid bias of one of'said local oscillator tubes, such that the ratio of the receiver sensitiveness to said noise voltages is continually maintained at its optimum value and retarding the action of said rectified portion in the control circuit in order to prevent the control action from being influenced by the keying frequency of received keying or modulation signals.

KURT KOSCHMIEDER. 

